1. Field of the Invention
The present invention relates to ultrasound transducers. In particular, the invention relates to ultrasound transducers mounted on catheters that include ultrasound dampening regions to improve the performance thereof.
2. Description of the Related Art
For certain types of minimally invasive medical procedures, endoscopic visualization of the treatment site within the body is unavailable or does not assist the clinician in guiding the needed medical devices to the treatment site.
Examples of such procedures are those used to diagnose and treat supra-ventricular tachycardia (SVT), atrial fibrillation (AF), atrial flutter (AFL) and ventricular tachycardia (VT). SVT, AFL, AF and VT are conditions in the heart which cause abnormal electrical signals to be generated, causing irregular beating of the heart.
A procedure for diagnosing and treating SVT or VT involves measuring the electrical activity of the heart using an electrophysiology (EP) catheter introduced into the heart via the patient's vasculature. The catheter carries mapping electrodes which are positioned within the heart and used to measure electrical activity. The position of the catheter within the heart is ascertained using fluoroscopic images. The mapping electrodes measure the electrical activity of the heart at the position of the catheter. A map is created by correlating locations in the heart determined by viewing the position of the catheter with the fluoroscope. The physician uses the map to identify the region of the endocardium which he believes to be the source of the abnormal electrical activity. An ablation catheter is then inserted through the patient's vasculature and into the heart where it is used to ablate the region identified by the physician.
To treat AF, an ablation catheter is maneuvered into the right or left atrium where it is used to create elongated ablation lesions in the heart.
An improvement over fluoroscopy is a display system using a fixed coordinate system for determining the relative locations of medical devices within the body. Such a display system using a fixed coordinate system can avoid the tracking errors inherent in fluoroscopic imaging that can make it difficult to guide medical devices to the desired locations within the body. Ultrasound can be used to track medical devices relative to a fixed internal coordinate system. Such an ultrasound tracking system uses at least four ultrasound transducers.
An ultrasound tracking system can be based on the time difference measured from the time an ultrasound pulse is transmitted by one transducer to the time it is received by another transducer. Given the velocity of sound in tissue and blood of approximately 1570 m/sec, the distance between the transmitter and receiver can be calculated. This process of distance measurement with sound is called sonomicrometry. Distance measurements between multiple transducers are used to triangulate the positions of the transducers in a three-dimensional coordinate system. A minimum of four transducers create a three-dimensional coordinate system, with three transducers defining a plane and the fourth defining a position above or below the plane. Additional transducers may be used for redundancy. Once the coordinate system is established, additional transducers on medical devices may be used to calculate the locations of the devices relative to the coordinate system.
FIG. 1 shows an example of transmit and receive waveforms for a typical sonomicrometry procedure. Transmission waveform 26 is a pulse initiated at time t1 by a transmitting transducer. Reception waveform 27 corresponds to the voltage generated by a receiving transducer that intercepts the transmit pulse. The time t2 is the time at which the reception waveform crosses a detection threshold 28. The time difference between t1 and t2 may then be used to calculate the distance between the transmitter and the receiver.
The detection threshold 28 is needed to filter out detected signals that are too small to have been generated by a measurement pulse. Such signals could result from crosstalk with signals on nearby wires or from random noise.
For optimal performance of a sonomicrometer system, transducers should exhibit fully isotropic operation, that is, the same transmit or receive (or both) generation or detection sensitivity in all directions. Isotropic operation allows measurement of the same distance amount regardless of the orientation of the transducers. Accurate distance measurement is desirable for measurements within the human body, especially within the heart or other vital organs, both for accurate placement of the sensing devices and for accurate location of areas for monitoring or surgery.
Sonometrics Corp. of London, Ontario, Canada has developed a sonomicrometer system based on a family of small transducers which might be sewn onto living tissue. These transducers typically consist of either small, flat squares of piezoelectric ceramic 30 (FIG. 2) in a spherical bead of epoxy (not shown) or small cylinders of piezoelectric ceramic 34 (FIG. 3) in a spherical bead of epoxy (not shown).
Piezoelectric ceramics convert electrical energy into vibrational energy, and vice versa. The vibration results from the piezoelectric ceramic expanding in one direction, which causes it to contract in another direction. For the square transducer 30, expansion by the top and bottom (thickness) causes the sides to contract. For the cylindrical transducer 34, expansion by the top and bottom (length) causes the thickness and the circumference to contract. Each direction of expansion/contraction is termed a vibrational mode.
The expansion and contraction causes an ultrasound signal to be emitted from each surface of the transducer when transmitting. Similarly, a received ultrasound signal causes a receiving transducer to expand and contract, generating electricity.
The epoxy bead serves as a lens to focus the ultrasound signal.
The frequency of expansion and contraction is determined by the size of the transducer. The speed of sound in the piezoelectric ceramic is about 4000 m/sec. This speed is equal to the wavelength times the frequency, and the wavelength is twice the direction of expansion/contraction. For example, for the square transducer 30, a length and width d of 0.052 inches relate to a frequency of 1.5 MHz. For the cylindrical transducer 34, the wavelength of the circumferential vibration is the average circumference, and the wavelength of the length vibration is twice the length, so a length of 0.052 inches and a circumference of 0.105 inches relate to a frequency of 1.5 MHz.
Line 40 in FIGS. 2 and 3 passes through the center of the transducer in a direction parallel to a longitudinal axis of a catheter (not shown) on which either of the transuducers 30 or 34 may be mounted. Line 38 indicates a direction perpendicular to line 40, where line 38 passes through the center of the transducer. The angle .THETA. denotes the angle from line 38 in the direction of line 40.
Unfortunately, the transducers shown in FIGS. 2 and 3 are anisotropic because the physical geometry of the transducer contributes to the radiation emitted and received. For example, referring to FIG. 3, line 38 lies in a radial plane perpendicular to line 40 through the center of transducer 34. The cylindrical symmetry of transducer 34 suggests that the emissions or receptions in this plane would also be symmetric. However, for measurements made at an angle .THETA. as the angle approaches line 40 (an angle of 90 degrees), defining a longitudinal plane through the center of the cylinder, anisotropy in emissions results as the radiation shifts from emission off the cylindrical side to emission off the flat end 44 of the transducer. Similarly, referring to FIG. 2, anisotropy results from interference between the sides and the top.
Furthermore, anisotropy results from the physical dimensions of a receiving transducer at the same angles. This is because reception is symmetric to transmission. For example, if an ideal emission source is moved around a stationary receiving transducer having a given geometric shape, this results in the same received waveform as when an ideal receiver is moved around a stationary transmitting transducer having the same geometric shape.
Measurements of the anisotropy of the devices shown in FIGS. 2 and 3 were on the order of 10 to 20 dB. Such anisotropy is undesirable because it disrupts accurate measurement of distances. For example, for a given distance d, measuring the distance between two transducers with isotropic radiation patterns will produce the same result regardless of the angular relationship between the two transducers. However, prior art transducers have anisotropic radiation patterns due to interference produced by emissions off the different surfaces. Such interference produces peaks and valleys in the radiated signal that vary based on the angle. Such peaks and valleys can occur at a time corresponding to the actual distance d, which causes the measured distance to vary because the wave may not be detected until after the interference minimum has passed. Such variation can be especially undesirable when performing the medical procedures described above, when accurate distance measurement is critical.
FIG. 4 illustrates the experimental results from catheters comprising small cylinders of piezoelectric ceramic around the catheter body. As can be seen, the amplitude of the signal (either transmitted or received) varies significantly with the angle .THETA.. Similar irregular results were observed with and without a spherical bead of epoxy around the ceramic. Furthermore, these devices typically would exhibit phase reversals in their response at a .THETA. of about 30 degrees, this effect being due to a switch from emission off the cylindrical surface 42 of the ceramic to emission off the ends 44 of the cylinder (see FIG. 3). Emission off the ends 44 is 180 degrees out of phase with emission off the surface 42 because one expands while the other contracts, causing interference. These responses were considered unacceptable.
FIG. 5 illustrates exemplary waveforms showing how the interference from anisotropic radiation may result in distance measurement variations. Transmitted waveform 50 is pulsed at time t1. Received waveform 52 rises above the detection threshold 54 at time t3. The actual time that the wave should be received is at time t2 (compare with FIG. 1). Before time t2 the wave looks like it is about to be detected, but the anisotropy results in destructive interference between time t2 and time t3. Thus, the distance measurement of the wave in FIG. 5 will be longer than the actual distance. Thus, anisotropy causes the distance measurement to vary based on the relative angle between the transmitter and the receiver.
FIGS. 6A and 6B show a second problem that results when a catheter 60 has both a transmitting transducer 62 and a receiving transducer 64. FIG. 6A shows that desirably, a measurement pulse generated by transmitting transducer 62 should be received by receiving transducer 64 at a time corresponding to the distance 66. However, due to the higher velocity of sound in the catheter materials than in the surrounding blood and tissues, the pulse may propagate more rapidly along the catheter body than through the surrounding blood and tissues. In such a situation the distance measured, signified by the numeral 68 in FIG. 6B, will be shorter than the actual distance. When distance 68 is used to triangulate the position of transducer 64, an error 70 results.
In accordance with the subject invention, the angular transmission and detection response of a transducer assembly can be improved if the geometry and structure of the device are carefully considered. The present invention achieves this result by employing ultrasound dampening regions adjacent to the transducers to produce more isotropic transmission and reception.
Sonic energy does not propagate in a vacuum. Ultrasonic energy does not propagate well in air nor do piezoelectric transducers couple energy effectively into air, especially at frequencies up to and beyond 1 MHz. Consequently an ultrasound dampening region including air or a vacuum is proposed to inhibit emission and reception from selected surfaces of the transducer.